2.1. Factors Influencing Neurite Growth in the Central Nervous System
Cell attachment, cell spreading, cell motility, and, in particular, neurite outgrowth are strongly dependent on cell-substrate interactions (Sanes, 1983, Ann. Rev. Physiol. 45:581-600; Carbonetto et al., 1987, J. Neurosci. 7:610-620). An increasing number of substrate molecules favoring neuroblast migration or neurite outgrowth have been found in central and peripheral nervous tissue (Cornbrooks et al., 1983, Proc. Natl. Acad. Sci. USA 80:3850-3854; Edelman, 1984, Exp. Cell Res. 161:1-16; Liesi, 1985, EMBO J. 4:1163-1170; Chiu, A. Y. et al., 1986, J. Cell Biol. 103:1383-1398; Fischer et al., 1986, J. Neurosci. 6:605-612; Lindner et al., 1986, Brain Res. 377:298-304; Mirsky et al., 1986, J. Neurocytol. 15:799-815; Stallcup et al., 1986, J. Neurosci. 5:1090-1101; Carbonetto et al., 1987, J. Neurosci. 7:610-620). The appearance of some of these factors can be correlated with specific developmental stages, and, in the peripheral nervous system (PNS), also with denervation (Edelman, 1984, Exp. Cell Res. 161:1-16; Liesi, 1985, EMBO J. 4:1163-1170; Stallcup et al., 1985, J. Neurosci. 5:1090-1101; Daniloff et al., 1986, J. Cell Biol. 103:929-945; Carbonetto et al., 1987, J. Neurosci. 7:610-620). The extracellular matrix protein tenascin has been shown to possess nonpermissive substrate properties (Chiquet-Ehrismann et al., 1986, Cell 47:131-139).
One of the most characterized of the soluble factors favoring neurite outgrowth is nerve growth factor (NGF). NGF promotes nerve fiber outgrowth from embryonic sensory and sympathetic ganglia in vivo and in vitro as well as neurite outgrowth (reviewed in Thoenen et al., 1982, In: Repair and Regeneration of the Nervous System, J. G. Nicholls, ed., Springer-Verlag, N.Y., pp. 173-185). NGF may also guide the direction of such neurite outgrowth. Three different molecular forms of NGF have been recognized. One type is a dimer (molecular weight .about.26,000) composed of two noncovalently linked, identical polypeptide chains. The second form is stable at neutral pH and contains three different polypeptide chains, .alpha., .beta. and .gamma. (molecular weight .about.140,000). The .beta. chain is the biologically active chain and is identical to the first form of NGF. The third form, which is isolated primarily from mouse L cells, (see U.S. Pat. No. 4,230,691, by Young, issued Oct. 28, 1980, and references therein) has a molecular weight of about 160,000 but is unstable at neutral pH. NGF has thus far been isolated from the submandibullar glands of mice, mouse L cells, and the prostate gland of the guinea pig and bull (reviewed in Thoenen et al., 1982, supra). No differences between the biological action of mouse, guinea pig and bull NGF have been detected. In addition, NGF isolated from mice have been found to bind to the human NGF receptor (Johnson et al., 1986, Cell 47:545-554).
The differentiated central nervous system (CNS) of higher vertebrates is capable of only very limited regenerative neurite growth after lesions. Limited regeneration after lesion has been seen in the retina (McConnell and Berry, 1982, Brain Res. 241:362-365) and in aminergic unmyelinated fiber tracts after chemical (Bjorklund and Stenevi, 1979, Physiol. Rev. 59:62-95) but not mechanical lesions (Bregman, 1987, Dev. Brain Res. 34:265-279).
Davis et al. (1987, Science 236: 1101-1109) and Gage et al. (19SS, Exp. Brain Res. 72: 371-380) observed a limited ingrowth of fibers into the hippocampus at 8-12 weeks. The longest growth of cholinergic fibers reinnervating the hippocampus was reported by Kromer et al. (1981, Brain Res. 210: 153-200) and Tuszynski et al. (1990, Neurosci. 36: 33-44), using embryonic tissue bridges. In these experiments, only short AchE-positive fibers were seen in the hippocampus at 4-6 weeks after operation. However, after very long survival times of 3-14 months, AchE-positive fibers were seen to extend over 2.5-3.5 mm. This very slow regeneration over longer distances was interpretated in the context of "glial reactions" in the denervated hippocampus (Kromer et al., 1981, Brain Res. 210: 153-200; Gage et al., 1988, Exp. Brain Res. 72: 371-380). Other studies have entailed direct grafting of cholinergic neurons into the denervated hippocampus (Bjorklund and Stenevi, 1984, Ann. Rev. Neurosci. 7: 279-308; Gage et al., 1987, in Progr. Brain Res., Seil et al. (eds.), Amsterdam: Elsevier, pp. 335-347, for review). Electrophysiological evidence for the reformation of septo-hippocampal connections across bridges has also been obtained (Segal et al., 1981, Neurosci. Letters 27: 7-12 . The finding that grafting of other types of embryonic cholinergic neurons gives rise to an aberrant cholinergic innervation (Nilsson et al., 1988, J. Comp. Neurol. 268: 204-222) shows a high level of specificity in guidance cues in the adult hippocampus.
Neurite growth from implanted embryonic CNS tissues in adult rat CNS has been found in some cases to reach up to 14 mm within some gray matter areas, but has not been found to exceed 1 mm within white matter (Nornes et al., 1983, Cell Tissue Res. 230:15-35; Bjorklund and Stenevi, 1979, Physiol. Rev. 59:62-95; Commission, 1984, Neuroscience 12:839-853). On the other hand, extensive regenerative growth has been found in the CNS of lower vertebrates and in the peripheral nervous system of all vertebrates including man.
Results from transplantation experiments indicate that the lack of regeneration is not an intrinsic property of CNS neurons, as these readily extend processes into implanted peripheral nervous tissue (Benfey and Aguayo, 1982, Nature (London) 296:150-152; Richardson et al., 1984, J. Neurocytol. 13:165-182 and So and Aguayo, 1985, Brain Res. 328:349-354). PNS neurons, however, failed to extend processes into CNS tissue, thus indicating the existence of fundamental differences between the two tissues (Aguayo et al., 1978, Neurosci. Lett. 9:97-104; Weinberg and Spencer, 1979, Brain Res. I62:273-279).
One major difference between PNS and CNS tissue is the differential distribution of the neurite outgrowth promoting extracellular matrix component laminin (Liesi, 1985, EMBO J. 4:2505-2511; Carbonetto et al., 1987, J. Neurosci. 7:610-620). Other factors though may be involved. Drastic differences have been observed in neurite growth supporting properties of sciatic and of optic nerve explants in vitro, in spite of the presence of laminin immunoreactivity in both explants (Schwab and Thoenen, 1985, J. Neurosci. 5:2415-2423). These experiments were carried out in the presence of optimal amounts of neurotrophic factors and differences persisted upon freezing of tested substrates.
It has been suggested that the differentiated CNS may lack cellular or substrate constituents that are conducive for neurite growth during development (Liesi, 1985, EMBO J. 4:2505-2511; and Carbonetto et al, 1987, J. Neurosci. 7:610-620), or it may contain components which are nonpermissive or inhibitory for nerve fiber regeneration (Schwab and Thoenen, 1985, J. Neurosci. 5:2415-2423).
Recently, a growth (cell proliferation) inhibitory factor for mouse neuroblastoma cells was partially purified and characterized from the culture medium of fetal rat glioblasts as well as from C6 rat glioma cells (Sakazaki et al., 1983, Brain Res. 262:125-135). The factor was estimated to have a molecular weight of about 75,000 by gel filtration with BioGel P-20 with an isoelectric point of 5.8. The factor did not appear to alter the growth rate or morphology of glial cells (C6) or fibroblasts (3T3). In addition, no significant nerve growth inhibitory factor activity was detected towards neuroblastoma cells (Neuro La, NS-20Y and NIE-115) or cloned fibroblasts (3T3).